Aluminum heat exchanger

ABSTRACT

A heat transfer system is disclosed that includes a heat transfer fluid circulation loop, and also a heat exchanger that includes an aluminum alloy exterior surface having thereon a top surface coat derived from a composition comprising a trivalent chromium salt and an alkali metal hexafluorozirconate.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein generally relates to heat exchangersand, more particularly, to aluminum heat exchangers that are resistantto corrosion.

Heat exchangers are widely used in various applications, including butnot limited to heating and cooling systems including fan coil units,heating and cooling in various industrial and chemical processes, heatrecovery systems, and the like, to name a few. Many heat exchangers fortransferring heat from one fluid to another fluid utilize one or moretubes through which one fluid flows while a second fluid flows aroundthe tubes. Heat from one of the fluids is transferred to the other fluidby conduction through the tube walls. Many configurations also utilizefins in thermally conductive contact with the outside of the tube(s) toprovide increased surface area across which heat can be transferredbetween the fluids, improve heat transfer characteristics of the secondfluid flowing through the heat exchanger and enhance structural rigidityof the heat exchanger. Such heat exchangers include microchannel heatexchangers and round tube plate fin (RTPF) heat exchangers.

Heat exchanger tubes may be made from a variety of materials, includingaluminum, copper and alloys thereof. Aluminum alloys are lightweight,have a relatively high specific strength and high heat conductivity. Dueto these excellent mechanical properties, aluminum alloys are used inheat exchangers for heating and cooling systems in commercial,industrial, residential, transport, refrigeration, and marineapplications. However, aluminum alloy heat exchangers can be susceptibleto corrosion. In applications in or close to marine environments,particularly, sea water or wind-blown seawater mist create an aggressivechloride environment that is detrimental for these heat exchangers. Thischloride environment rapidly causes localized and general corrosion ofbraze joints, fins, and refrigerant tubes. The corrosion modes includegalvanic, crevice, and pitting corrosion. Corrosion impairs the heatexchanger ability to transfer heat, as fins lose their structuralintegrity and contact with the refrigerant tubes and corrosion productsaccumulate on the heat exchanger external surfaces creating an extrathermal resistance layer and increasing airflow impedance. Corrosioneventually leads to a loss of refrigerant due to tube perforation andfailure of the cooling system. Accordingly, improvements in corrosiondurability of aluminum alloy heat exchangers would be well received inthe art.

Surface coatings have been used to provide protection against corrosionby imposing a physical barrier between salt water in the environment andaluminum components of the heat exchanger. Coating types includeelectroplating, dip coating, spray coating and powder coating. However,conventional polymer surface coatings can suffer from a number ofproblems such as inadequate or uneven thickness, pinholes and other gapsin coating coverage, and the necessity of extensive surface preparationof the aluminum substrate prior to application of the coating in orderto provide adequate bonding between the coating and the substrate, inaddition to the cost, time and complexity of applying the polymercoating. Heat exchangers, by their nature, exhibit large and frequenttemperature variations, which can lead to the delamination anddisbanding of the polymer coatings. Furthermore, polymer coatings createa layer which is resistive to heat transfer and can create a loss inefficiency for the heat exchanger. Metal surface treatments such as TCP(trivalent chromium process developed by the U.S. Naval Air WarfareCenter Aircraft Division) have been used to prepare heat exchangersurfaces for subsequent application of polymer coatings as described inUS patent application publication no. 2012/0183755 A1; however, suchcoatings are still subject to the issues described above.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a heat transfer systemcomprises a heat transfer fluid circulation loop. The heat transfersystem includes a heat exchanger disposed in the heat transfer fluidcirculation loop, the heat exchanger comprising an aluminum alloyexterior surface having thereon a top surface coat derived from acomposition comprising a trivalent chromium salt and an alkali metalhexafluorozirconate.

According to another aspect of the invention, a method of producing aheat transfer system comprises contacting an aluminum alloy exteriorsurface of a heat exchanger with a composition comprising a trivalentchromium salt and an alkali metal hexafluorozirconate to form a topsurface coat on the aluminum alloy surface, and assembling the heatexchanger comprising the top surface coat into a heat transfer fluidcirculation loop.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawing in which:

FIG. 1 depicts a schematic diagram of an exemplary heat transfer system;

FIG. 2 depicts a schematic diagram of an exemplary heat exchanger;

FIG. 3 depicts a schematic diagram of another exemplary heat exchanger;and

FIG. 4 depicts a schematic diagram of a cross-sectional view of analuminum alloy surface of a heat exchanger.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the Figures, an exemplary heat transfer system with aheat transfer fluid circulation loop is shown in block diagram form inFIG. 1. As shown in FIG. 1, a compressor 10 pressurizes heat transferfluid in its gaseous state, which both heats the fluid and providespressure to circulate it throughout the system. The hot pressurizedgaseous heat transfer fluid exiting from the compressor 10 flows throughconduit 15 to condenser heat exchanger 20, which functions as a heatexchanger to transfer heat from the heat transfer fluid to thesurrounding environment, resulting in condensation of the hot gaseousheat transfer fluid to a pressurized moderate temperature liquid. Theliquid heat transfer fluid exiting from the condenser 20 flows throughconduit 25 to expansion valve 30, where the pressure is reduced. Thereduced pressure liquid heat transfer fluid exiting the expansion valve30 flows through conduit 35 to evaporator heat exchanger 40, whichfunctions as a heat exchanger to absorb heat from the surroundingenvironment and boil the heat transfer fluid. Gaseous heat transferfluid exiting the evaporator 40 flows through conduit 45 to thecompressor 10, thus completing the heat transfer fluid loop. The heattransfer system has the effect of transferring heat from the environmentsurrounding the evaporator 40 to the environment surrounding thecondenser 20. The thermodynamic properties of the heat transfer fluidallow it to reach a high enough temperature when compressed so that itis greater than the environment surrounding the condenser 20, allowingheat to be transferred to the surrounding environment. The thermodynamicproperties of the heat transfer fluid must also have a boiling point atits post-expansion pressure that allows the environment surrounding theevaporator 40 to provide heat at a temperature to vaporize the liquidheat transfer fluid.

The heat transfer system shown in FIG. 1 can be used as an airconditioning system, in which the exterior of condensr heat exchanger 20is contacted with air in the surrounding outside environment and theevaporator heat exchanger 40 is contacted with air in an interiorenvironment to be conditioned. Additionally, as is known in the art, thesystem can also be operated in heat pump mode using a standard multiportswitching valve to reverse heat transfer fluid flow direction and thefunction of the condenser and evaporator heat exchangers, i.e. thecondenser in a cooling mode being evaporator in a heat pump mode and theevaporator in a cooling mode being the condenser in a heat pump mode.Additionally, while the heat transfer system shown in FIG. 1 hasevaporation and condensation stages for highly efficient heat transfer,other types of heat transfer fluid loops are contemplated as well, suchas fluid loops that do not involve a phase change, for example,multi-loop systems such as commercial refrigeration or air conditioningsystems systems where a non-phase change loop thermally connects one ofthe heat exchangers in an evaporation/condensation loop like FIG. 1 to asurrounding outside environment or to an interior environment to beconditioned. Regardless of the specific configuration of the heattransfer fluid circulation loop, a heat exchanger may be disposed in apotentially corrosive environment such as a marine or ocean shoreenvironment.

One type of exemplary heat exchanger that can be used according to theembodiments described herein is a micro-channel or mini-channel heatexchanger. The configuration of these types of heat exchangers isgenerally the same, with the primary difference being rather looselyapplied based on the size of heat transfer tube ports. For the sake ofconvenience, this type of heat exchanger will be referred to herein as amicro-channel heat exchanger. As shown in FIG. 2, a micro-channel heatexchanger 20 includes first manifold 212 having inlet 214 for receivinga working fluid, such as coolant, and outlet 216 for discharging theworking fluid. First manifold 212 is fluidly connected to each of aplurality of tubes 218 that are each fluidly connected on an oppositeend with second manifold 220. Second manifold 220 is fluidly connectedwith each of a plurality of tubes 222 that return the working fluid tofirst manifold 212 for discharge through outlet 216. Partition 223 islocated within first manifold 212 to separate inlet and outlet sectionsof first manifold 212. Tubes 218 and 222 can include channels, such asmicrochannels, for conveying the working fluid. The two-pass workingfluid flow configuration described above is only one of many possibledesign arrangements. Single and other multi-pass fluid flowconfigurations can be obtained by placing partitions 223, inlet 214 andoutlet 216 at specific locations within first manifold 212 and secondmanifold 220.

Fins 224 extend between tubes 218 and the tubes 222 as shown in theFigure. Fins 224 support tubes 218 and tubes 222 and establish open flowchannels between the tubes 218 and tubes 222 (e.g., for airflow) toprovide additional heat transfer surfaces and enhance heat transfercharacteristics. Fins 224 also provide support to the heat exchangerstructure. Fins 224 are bonded to tubes 218 and 222 at brazed joints226. Fins 224 are not limited to the triangular cross-sections shown inFIG. 2, as other fin configurations (e.g., rectangular, trapezoidal,oval, sinusoidal) can be used as well. Fins 224 may have louvers toimprove heat transfer.

Referring now to FIG. 3, an exemplary RTPF (round tube plate fin) heatexchanger is shown. As shown in FIG. 3, a heat exchanger 20 includes oneor more flow circuits for carrying refrigerant. For the purposes ofexplanation, the heat exchanger 20 is shown with a single flow circuitrefrigerant tube 320 consisting of an inlet line 330 and an outlet line340. The inlet line 330 is connected to the outlet line 340 at one endof the heat exchanger 20 through a 90 degree tube bend 350. It should beevident, however, that more circuits may be added to the unit dependingupon the demands of the system. For example, although tube bend 350 isshown as a separate component connecting two straight tube section, thetube 320 can also be formed as a single tube piece with a hairpinsection therein for the tube bend 350, and multiple units of suchhairpin tubes can be connected with u-shaped connectors at the open endsto form a continuous longer flow path in a ‘back-and-forth’configuration. The heat exchanger 20 further includes a series of fins360 comprising radially disposed plate-like elements spaced along thelength of the flow circuit, typically connected to the tube(s) 320 withan interference fit. The fins 360 are provided between a pair of endplates or tube sheets 370 and 380 and are supported by the lines 330,340 in order to define a gas flow passage through which conditioned airpasses over the refrigerant tube 320 and between the spaced fins 360.Fins 360 may include heat transfer enhancement elements such louvers.

The refrigerant tubes can be made of an aluminum alloy based corematerial and, in some embodiments, may be made from aluminum alloysselected from 1000 series, 3000 series, 5000 series, or 6000 seriesaluminum alloys. The fins can be made of an aluminum alloy substratematerial such as, for example, materials selected from the 1000 series,3000 series, 6000 series, 7000 series, or 8000 series aluminum alloys.The embodiments described herein utilize an aluminum alloy for the finsof a tube-fin heat exchanger having an aluminum alloy tube, i.e., aso-called “all aluminum” heat exchanger. In some embodiments, componentsthrough which refrigerant flows, such as tubes and/or manifolds, can bemade of an alloy that is electrochemically more cathodic than connectedcomponents through which refrigerant does not flow (e.g., fins). Thisensures that any galvanic corrosion will occur in non-flow-throughcomponents rather than in flow-through components, in order to avoidrefrigerant leaks.

As mentioned above, heat exchanger component connections, such asbetween tubes and fins, or between tubes and manifolds, can be connectedby brazing. Brazing compositions for aluminum components are well-knownin the art as described, for example, in U.S. Pat. Nos. 4,929,511,5,820,698, 6,113,667, and 6,610,247, and US published patent application2012/0170669, the disclosures of each of which are incorporated hereinby reference in their entirety. Brazing compositions for aluminum caninclude various metals and metalloids, including but not limited tosilicon, aluminum, zinc, magnesium, calcium, lanthanide metals, and thelike. In some embodiments, the brazing composition includes metals moreelectrochemically anodic than aluminum (e.g., zinc), in order to providesacrificial galvanic corrosion in the braze joint(s) instead of therefrigerant tube(s).

A flux material can be used to facilitate the brazing process. Fluxmaterials for brazing of aluminum components can include high meltingpoint (e.g., from about 564° C. to about 577° C.), such as LiF and/orKAlF₄. Other compositions can be utilized, including cesium, zinc, andsilicon. The flux material can be applied to the aluminum alloy surfacebefore brazing, or it can be included in the brazing composition. Afterthe brazing is complete, any flux residue can be removed prior tocontact with the trivalent chromium composition, but it does not have tobe. Therefore, in some embodiments, the flux material is not removedprior to contact with the trivalent chromium composition.

In some embodiments, a metal more electrochemically anodic thanaluminum, such as zinc, can be applied to a surface of the heatexchanger before brazing and before contact with the trivalent chromiumcomposition. Various techniques can be used to apply the anodic metal,such as electrodeposition, physical vapor deposition, or various methodsof thermal spray such as plasma spray, flame spray, cold spray, HVOF,and other known thermal spray techniques. Alternatively, a layer of zincor zinc powder can be physically applied to the surface and then heated,as is known in the art. This anodic layer can be thermally diffused intothe aluminum substrate, e.g., to a depth of 80-100 μm. The applicationof a top surface coat of trivalent chromium composition acts to enhancethe protection of this anodic layer.

As described herein, a surface of the heat exchanger has a top surfacecoat on at least a portion thereof derived from a composition comprisinga trivalent chromium salt and an alkali metal hexafluorozirconate. Suchcompositions, along with methods for applying to metal surfaces aredescribed in detail in U.S. Pat. Nos. 6,375,726, 6,511,532, 6,521,029,and 6,511,532, the disclosure of each of which is incorporated herein byreference in its entirety.

The trivalent chromium salt can contain various anions along with thetrivalent chromium. Exemplary anions include nitrate, sulfate,phosphate, and/or acetate. Specific exemplary trivalent chromium saltscan include Cr₂(SO₄)₃, (NH)₄Cr(SO₄)₂, KCr(SO₄)₂, and mixtures comprisingany of the foregoing. The concentration of the trivalent chromium saltin the composition, per liter of solution, can range from about 0.01 gto about 22 g, more specifically from about 3 g to about 12 g, and evenmore specifically from about 4 g to about 8.0 g.

The alkali metal hexafluorozirconate can contain various cations such aspotassium or sodium. The concentration of alkali metalhexafluorozirconate, per liter of solution, can range from about 0.01 gto about 12 g, more specifically from about 6 g to about 10 g.

In some embodiments, the composition can also comprise an alkali metaltetrafluoroborate and/or an alkali metal hexafluorosilicate, such aspotassium or sodium tetrafluoroborate, or potassium or sodiumhexafluorosilicate. The concentration of the alkali metaltetrafluoroborate and/or an alkali metal hexafluorosilicate, per literof solution, can range from about 0.01 g to about 12 g, morespecifically from about 6 g to about 10 g.

As mentioned above, the trivalent chromium composition is acidic. Morespecifically, the composition can be an acidic aqueous solution having apH ranging from about 2 to about 6, more specifically from about 2.5 toabout 4.5, and even more specifically from about 3.7 to about 4.0.Acidity can be provided by incorporating the acid of the trivalentchromium salt, or alternatively by any known acid such as sulfuric acid,nitric acid, phosphoric acid, and/or acetic acid.

In some embodiments, the composition optionally comprises a watersoluble thickener. When present, a water-soluble thickener such ascellulose derivatives, starches, and/or, soluble gums can be present inthe acidic solution in amounts ranging from about 0.1 g to about 10 gper liter, more specifically from about 0.1 g to about 2.0 g, even morespecifically from about 0.5 g to about 2.0 g, and even more specificallyfrom about 0.5 g to about 1.5 g, s liter of the aqueous solution.Specific examples of thickeners include the cellulose compounds, e.g.hydroxypropyl cellulose, ethyl cellulose, hydroxyethyl cellulose,colloidal silica, clays, starches, gums, and various combinationsthereof.

In some embodiments, the composition optionally comprises awater-soluble surfactant. When present, a surfactant can be present inthe acidic solution in amounts ranging from about 0.1 g to about 10 gper liter, more specifically from about 0.1 g to about 2.0 g, even morespecifically from about 0.5 g to about 2.0 g, and even more specificallyfrom about 0.5 g to about 1.5 g, per liter of the aqueous solution.These surfactants are known in the art of aqueous solutions and includeorganic compounds that are non-ionic, cationic, and/or anionicsurfactants. Exemplary surfactants include the monocarboxylimidoazoline, alkyl sulfate sodium salts (DUPONOL®), tridecyloxypoly(alkyleneoxy ethanol), ethoxylated or propoxylated alkyl phenol(IGEPAL®.), alkyl sulfoamides, alkaryl sulfonates, palmitic alkanolamides (CENTROL®), octylphenyl polyethoxy ethanol (TRITON®), sorbitanmonopalmitate (SPAN®), dodecylphenyl polyethylene glycol ether (e.g.TERGITROL®), alkyl pyrrolidone, polyalkoxylated fatty acid esters,alkylbenzene sulfonates and mixtures thereof. Other known water solublesurfactants are disclosed by “Surfactants and Detersive Systems”,published by John Wiley & Sons in Kirk-Othmer's Encyclopedia of ChemicalTechnology, 3^(rd) Ed.

The treatment composition can be applied by any of a number of knowncoating techniques, including dip coating, spray coating, brush coating,roll coating, etc. The composition can be applied to only a portion ofthe aluminum alloy surface(s) of the heat exchanger, e.g., thoseparticularly susceptible to corrosion, such as u-shaped tubes or hairpintube sections of a RTPF heat exchanger, or it can be applied to theentire surface of the heat exchanger. In some embodiments, dip coatingis effectively used to cover the entire surface of the heat exchanger.In some embodiments, agitation of either the heat exchanger work-piecein the coating solution or of the liquid solution itself (e.g., withjets or mechanical agitation) is used during dip coating. In someembodiments, a microchannel heat exchanger is oriented with the tubesgenerally vertical during dip coating or during removal from the coatingbath. In some embodiments, a RTPF heat exchanger is oriented with thetubes generally horizontal during dip coating or during removal from thecoating bath. The post treatment of the metal coating can be carried outat temperatures ranging from ambient temperatures, e.g., 20° C. or 25°C., up to about 65° C. The duration for which the composition iscontacted with the aluminum alloy before subsequent processing such asrinsing and/or drying can range widely. Exemplary contact times canrange from 5 to 15 minutes, more specifically from 9 to 11 minutes, andeven more specifically about 10 minutes. The coating may be air dried atambient conditions, or can be subject to accelerated drying by any ofthe methods known in the art, for example, oven drying, forced airdrying, exposure to infra-red lamps, etc. Exemplary drying conditionsinclude about 24 hours at room temperature and less than 50% relativehumidity, or about 2 hours at 50. The resulting coating as applied bythe methods described above produces a mixed-metal oxide layer ofapproximately 50-100 nm in thickness permanently integrated with thealuminum alloy. Heat transfer calculations indicate that thermalresistance of this layer is negligible relative to the traditionalorganic coatings. The coated aluminum surface is schematically depictedin FIG. 4, which shows a cross sectional view of aluminum alloy 410having a top surface coat of mixed metal oxide layer 420.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

1. A heat transfer system comprising a heat transfer fluid circulationloop, comprising a heat exchanger disposed in said heat transfer fluidcirculation loop, the heat exchanger comprising an aluminum alloyexterior surface having thereon a top surface coat on at least a portionof the heat exchanger derived from a composition comprising a trivalentchromium salt and an alkali metal hexafluorozirconate in an aqueoussolution.
 2. The heat transfer system of claim 1, wherein the heatexchanger comprises a first aluminum alloy component connected bybrazing to a second aluminum alloy component.
 3. The heat transfersystem of claim 2, wherein the top surface coat is disposed over thefirst aluminum alloy component, the second aluminum alloy component, andthe brazing connecting the first and second aluminum alloy components.4. The heat transfer system of claim 2, wherein the brazing compriseszinc.
 5. The heat transfer system of claim 2, wherein the brazingincludes residue of a brazing flux on the surface thereof, comprising ametal salt with a melting point of 564° C. to 577° C.
 6. The heattransfer system of claim 5, wherein the metal salt comprises LiF and/orKAlF₄.
 7. The heat transfer system of claims 1, wherein said aluminumalloy exterior surface includes zinc enrichment of the aluminum alloysurface below said top surface coat.
 8. (canceled)
 9. (canceled) 10.(canceled)
 11. The heat transfer system of claim 1, wherein the entiresurface of the heat exchanger is covered by said top surface coat. 12.The heat transfer system of claim 1, wherein the heat exchanger is around tube plate fin heat exchanger, and only return bend tube portionsof the round tube plate fin heat exchanger are covered by said topsurface coat.
 13. A method of producing a heat transfer system,comprising: contacting an aluminum alloy exterior surface of a heatexchanger with a composition comprising a trivalent chromium salt and analkali metal hexafluorozirconate in an aqueous solution to form a topsurface coat on the aluminum alloy surface; and assembling the heatexchanger comprising said top surface coat into a heat transfer fluidcirculation loop.
 14. The method of claim 13, wherein the heat exchangercomprises a first aluminum alloy component connected by brazing to asecond aluminum alloy component.
 15. The method of claim 14, wherein thetop surface coat is disposed over the first aluminum alloy component,the second aluminum alloy component, and the brazing connecting thefirst and second aluminum alloy components.
 16. The method of claim 14,wherein the brazing comprises zinc.
 17. The method of claim 14, whereinthe brazing includes residue of a brazing flux on the surface thereof,comprising a metal salt with a melting point of 564° C. to 577° C. 18.The method of claim 17, wherein the metal salt comprises LiF and/orKAlF₄.
 19. The method of claim 13, wherein said aluminum alloy exteriorsurface includes zinc enrichment of the aluminum alloy surface belowsaid top surface coat.
 20. (canceled)
 21. (canceled)
 22. (canceled) 23.The method of claim 13, wherein said composition is applied by dipcoating, spray coating, brush coating, or a combination comprising oneor more of the foregoing.
 24. The method of claim 23, wherein saidcomposition is applied by dip coating.
 25. The method of claim 13,wherein the entire surface of the heat exchanger is covered by said topsurface coat.
 26. The heat transfer system of claim 13, wherein the heatexchanger is a round tube plate fm heat exchanger, and only return bendtube portions of the round tube plate fin heat exchanger are covered bysaid top surface coat.